1. Field
The present invention relates to image forming apparatuses and process cartridges, and in particular to image forming apparatuses and process cartridges that include and serve as imaging devices for forming images in black and for forming images in other colors, the imaging devices for forming images in black and for other colors each including a latent image carrier, a charging unit that charges the surface of the latent image carrier, and a lubricant applying unit that applies lubricant to the surface of the latent image carrier.
2. Background Art
Latent image carriers included in imaging devices need to be uniformly charged by respective charging units so as to have latent images formed thereon. The charging units each includes a charging member such as a charging wire, a roller, or a brush. Other examples of the charging member include a non-contact charging member that charges the surface of the latent image carrier spaced apart therefrom, and a contact charging member that charges the surface of the latent image carrier being in contact therewith. Charging biases to be applied to such a charging member also vary, such as a bias that includes only a direct current (DC) component, and a bias that includes a DC component and an alternating current (AC) component superposed thereon.
Charging units employing a charging wire as the charging member are not in contact with the surfaces of the latent image carriers and employ the DC charging method in which a charging bias including only a DC component is applied. Such charging units employing the DC charging method cause no hazard to the surfaces of the latent image carriers and therefore are advantageous in preventing surface peeling of the latent image carriers and in extending the lifetimes of the latent image carriers. Moreover, since such charging units are of the non-contact type, contaminants on the latent image carriers do not adhere to their charging members. However, there is a problem in that such charging units tend to generate ozone and nitrogen oxides (NOx) during charging.
Examples of a charging member in the form of a charging roller are classified into the following: a non-contact charging roller type in which the surface of the latent image carrier is charged without being in contact with the roller, and a contact charging roller type in which the surface of the latent image carrier is charged while being in contact with the roller. Non-contact charging rollers apply a charging bias by the AC+DC charging method in which a DC component and an AC component superposed thereon are applied. Contact charging rollers apply a charging bias by either the DC charging method in which only a DC component is applied or the AC+DC charging method in which a DC component and an AC component superposed thereon are applied.
Charging units of the non-contact charging roller type generate less ozone and NOx during charging than charging units of the charging wire type. In addition, contaminants on the latent image carriers infrequently adhere to the non-contact charging members, i.e., the charging rollers. However, there is a problem in that superposition of an AC component is highly hazardous to the surface of each latent image carrier. This increases the probability of surface peeling of the latent image carrier and thus shortens the lifetime of the latent image carrier.
Charging units employing the contact charging roller and the DC charging method have a simple configuration and are not hazardous to the surfaces of the latent image carriers. Therefore, such charging units are advantageous in preventing surface peeling of the latent image carriers and in extending the lifetimes of the latent image carriers. However, because of direct contact with the latent image carriers, contaminants on the latent image carriers easily adhere to the surfaces of the respective charging rollers. The contaminated portion of each charging roller may cause irregular charging leading to deterioration in image quality. In contrast, charging units employing the contact charging roller and the AC+DC charging method less frequently cause irregular charging than charging units employing the contact charging roller and the DC charging method, even if the charging roller is contaminated, because of the superposition of an AC component. However, the superposition of an AC component is highly hazardous to the surface of each latent image carrier. This increases the probability of surface peeling of the latent image carriers and thus shortens the lifetimes of the latent image carriers.
Charging units of imaging devices included in a full-four-color image forming apparatus have the following characteristics according to their configurations. In the case where a single imaging device includes a charging wire as the charging member, the amount of ozone to be generated is smaller than that in the case where all of the four imaging devices each includes a charging wire. Therefore, emission of ozone from the inside to the outside of the image forming apparatus can be suppressed. In this case where a single imaging device includes a charging wire, the amount of NOx to be generated is also smaller and thus the probability of occurrence of image distortion is smaller than in the case where all of the four imaging devices each includes a charging wire. Therefore, adverse influence on images can be suppressed to a negligible level. On the other hand, when a charging unit employing the contact charging roller and the DC charging method is applied to an imaging device that forms a black toner image (hereinafter referred to as an imaging device for black), image irregularity due to contamination of the charging roller is not noticeable and can be ignored in practical use. However, when the same charging unit is applied to imaging devices that form color toner images (hereinafter referred to as imaging devices for color), image irregularity due to contamination of the charging rollers is noticeable and may cause a problem in practical use.
When a charging unit employing the contact or non-contact charging roller and the AC+DC charging method is applied to imaging devices for color, which are used infrequently, the short lifetimes of the latent image carriers do not matter very much. However, when the same charging unit is applied to an imaging device for black, which is used frequently, the short lifetime of the corresponding latent image carrier may cause a problem in practical use.
To summarize, application of charging units having the same configuration to all of the imaging devices for four colors in a full-color image forming apparatus may cause problems in practical use. To solve such problems, there are some known image forming apparatuses in which at least one of a plurality of imaging devices includes a charging unit having a configuration different from those of the charging units included in the other imaging devices.
For example, in an image forming apparatus, a charging unit employing a charging wire is applied to the imaging device for black, and a charging unit employing a non-contact charging roller and the AC+DC charging method is applied to the imaging devices for color. Since only one of the charging units employs a charging wire, emission of ozone and image distortion caused by NOx are suppressed while the latent image carrier of the imaging device for black, which is used frequently, can be provided with a sufficient lifetime. In addition, since the charging unit employing the non-contact charging roller and the AC+DC charging method is only applied to the imaging devices for color, which are used infrequently, the problem that the lifetimes of the latent image carriers may be shortened because of the AC component that is hazardous thereto is negligible in practical use.
Moreover, since both charging units are of the non-contact type, contaminants on the latent image carriers infrequently adhere to the charging rollers. Accordingly, there is practically no problem due to contamination of the charging rollers.
Further, when a charging unit employing the contact charging roller and the DC charging method is applied to an imaging device for black, image irregularity due to contamination of the corresponding charging roller is not noticeable. Therefore, it is also allowable that a charging unit employing the contact charging roller and the DC charging method is applied to the imaging device for black while a charging unit employing the non-contact charging roller and the AC+DC charging method is applied to the imaging devices for color. That is, by applying charging units having different configurations to imaging devices according to the characteristics of the imaging devices, an image forming apparatus that infrequently has problems in practical use can be realized.
Under such circumstances, there is a known image forming apparatus in which lubricant is applied to the surfaces of latent image carriers from the viewpoints of suppressing surface peeling of the latent image carriers, improving removability of post-transfer residual toner on the latent image carriers, and preventing strong adhesion of toner components to the surfaces of the latent image carriers, which is called filming.
Even in the case where lubricant is applied to the surfaces of the latent image carriers as described above while charging units of the contact or non-contact charging roller type are used, a small amount of lubricant may adhere to the charging rollers. Consequently, irregular charging may occur at some portions of the charging rollers after a certain period of use, leading to deterioration in image quality.
Although color image forming apparatuses have become popular in recent years, the situation where color image forming apparatuses are always used in a full color mode is not so common yet, practically. Black-and-white images are still used in most cases. Therefore, among imaging devices for forming images in a plurality of colors, the imaging device for black has the shortest lifetime. Naturally, users are eagerly demanding that the lifetime of the imaging device for black be extended. In response to such a demand, an image forming apparatus including a DC charger in the imaging device for black and AC+DC charging rollers in the imaging devices for color has been realized and is becoming popular.
In such a case, use of a DC charger for black is less hazardous to a photoconductor, extends the lifetime of a cleaning blade, and reduces the occurrence of failure due to contamination caused by a lubricant applying unit. This is because the DC charger is spaced apart farther from the image carrier and therefore is less frequently contaminated than in the case of an AC+DC charging roller.
In addition, use of AC+DC charging rollers for color suppresses generation of ozone and reduces the space, such as an airflow path, necessary for ozone management, leading to size reduction of the image forming apparatus. Consequently, the imaging device for black has a longer lifetime than the imaging devices for color. Thus, an image forming apparatus that matches the current trend in which black-and-white images are used frequently can be provided.
Further, use of a DC charger for black is less hazardous to the photoconductor than use of AC+DC charging rollers for color. Therefore, the amount of lubricant to be applied by the lubricant applying unit can be reduced. This means that the lubricant applying unit for black having lubricant of the same amount as those of the lubricant applying units for color can be used longer than those for color.
Considering such circumstances, there is another example in which the amounts of lubricant to be applied by respective lubricant applying units are intentionally made different for the case for black and the case for color according to the charging method.
When different charging methods are employed for the imaging device for black and the imaging devices for color as described above with the proviso that dedicated process cartridges, serving as imaging devices, are used respectively for black and for color, the production cost will be increased.